GUI control, fwrite, and file control

Hi folks,

Pardon my naivete, but I'm having trouble with a GUI. I want to adjust a slider bar, and then fwrite the new value of the slider bar to a file. First, what works.

I can get a file identifier in a function, and pass that identifier to another function to do the actual writing, and then return control to the first file.

function[] = filecreate()             
  a=fopen('b.txt','at');                
  filewrite(a);                        
  fclose(a);
end

function[] = filewrite(b)
fwrite(b, 'HI');
end

So, as proof of concept, I should be able to pass the file identifier between functions. Now, onto the sliderbar GUI. I can write to a file if I open, write to, and close a file all within the slider bar callback function.

However, if I open a file outside of the callback and pass the file identifier to the callback, then I can't seem to write to the file.

I get the following error:

??? Error using ==> fwrite -Invalid file identifier. Use fopen to generate a valid file identifier. Error in ==> practice_slider>ed_call at 32 -fwrite(Speed_temp.file, 'H'); ??? Error while evaluating uicontrol Callback

Here is the function calling the GUI

function[] = slidingbarcontrol()
fid=fopen('a.txt','at');
practice_slider(fid);
fclose(fid);

And, here is the GUI.

function[] = practice_slider(file_id)
Speed.file=file_id;
fwrite(Speed.file,'Hello'); * %This successfully writes to the file*
Speed.last=uint8(0);
Speed.fh = figure('units','pixels',...
              'position',[300 300 300 100],...
              'menubar','none',...
              'name','Speed',...
              'numbertitle','off',...
              'resize','off');
Speed.sl = uicontrol('style','slide',...
                 'unit','pix',...
                 'position',[20 10 260 30],...
                 'min',0,'max',100,'val',0);             
Speed.ed = uicontrol('style','text',...
                 'unit','pix',...
                 'position',[20 50 260 30],...
                 'fontsize',16,...
                 'string','0');

set([Speed.ed,Speed.sl],'call',{@ed_call,Speed});

function [] = ed_call(varargin) %slider callback
[h,Speed_temp] = varargin{[1,3]}; 
fwrite(Speed_temp.file, 'H'); *%This fwrite causes error.*
switch h

Does anyone have any thoughts??? Thanks much. Tom

Answer 1

[Sean de Wolski]

The error is occuring because you don't have a uiwait/uiresume in your GUI to pause execution. Thus the file opens, the gui opens, the file closes, you slide and get the error. To fix it, pause execution by intserting a uiwait() at the end of the GUI and a uiresume when you close it.

doc uiwait
doc uiresume
doc waitfor %another approach

Answer 2

[T Fab]

Sean, thanks very much. Your comments made me go back and re-examine just what was going on with my program. I deleted the command to close the output file in the function slidingbarcontrol(), and added a pushbutton to the practice_slider GUI that, when clicked, will fclose the file, and delete the GUI. I put the corrected (and newly functioning) version here so if someone needs to see the implemented final answer, they have it available. Thanks again. Tom

function[] = slidingbarcontrol()
fid=fopen('a.txt','at');
practice_slider(fid);
%fclose(fid);    %This line is now deleted.
end

function[] = practice_slider(file_id)
Speed.file=file_id;  
Speed.last=uint8(0);
Speed.fh = figure('units','pixels',...
              'position',[300 300 300 100],...
              'menubar','none',...
              'name','Speed',...
              'numbertitle','off',...
              'resize','off');
Speed.pb = uicontrol('style','push',...     %I added this pushbutton
              'unit', 'pix',...             %to close the GUI and
              'position', [225 70 75 30],...%also close the output file.
              'string', 'Push to Close');
Speed.sl = uicontrol('style','slide',...
                 'unit','pix',...
                 'position',[20 10 260 30],...
                 'min',0,'max',100,'val',0);             
Speed.ed = uicontrol('style','text',...
                 'unit','pix',...
                 'position',[110 50 80 30],...
                 'fontsize',16,...
                 'string','0');

set([Speed.pb, Speed.ed,Speed.sl],'call',{@ed_call,Speed});

function [] = ed_call(varargin) %slider callback
[h,Speed_temp] = varargin{[1,3]}; 
fwrite(Speed_temp.file, 'H'); %This works now.
switch h  
    case Speed_temp.ed
        L = get(Speed_temp.sl,{'min','max','value'}); 
        E = uint8(str2double(get(h,'string'))); 
        if E >= L{1} && E <= L{2}
            set(Speed_temp.sl,'value',E) 
        else
            set(h,'string',L{3}) 
        end
    case Speed_temp.sl
        new_speed=uint8(get(h,'value'));
        set(Speed_temp.ed,'string',new_speed) 
        if(new_speed>Speed_temp.last)
            fwrite(file_id, 'F');  %Now I can fwrite here.
        elseif(Speed_temp.last>new_speed)
            fwrite(file_id, 'S');  %Now I can fwrite here.
        else
        end
        fwrite(file_id, 'DONE');   %Now I can fwrite here.
        Speed_temp.last=new_speed;
        set([Speed_temp.ed,Speed_temp.sl],'call',{@ed_call,Speed_temp});  
    case Speed_temp.pb
        fclose(Speed_temp.file);   %This will now close the output file.
        close(gcbf);               %This will close the GUI.
    otherwise
end